Welcome Jens,

I would like to know if and when it would be possible to have a texture on the top of the wall instead of only a colour ?

It's not possible. Maybe in a future version, but I don't see the real need.

Workaround ?

You could place a thin textured box on top of the wall or maybe even use a box instead of wall.

I would like to have some "hanging walls". Meaning I would like an elevation feature available for the walls. Is this something that is coming ?

This won't happen. If you want to elevate a wall, add it to a separate level at a higher elevation or make a hole at its bottom by adding a service hatch or even a hole of a different shape like in this tip.